Update fusion cost model + some additional infrastructure and debug information for -loop-fusion

- update fusion cost model to fuse while tolerating a certain amount of redundant
  computation; add cl option -fusion-compute-tolerance
  evaluate memory footprint and intermediate memory reduction
- emit debug info from -loop-fusion showing what was fused and why
- introduce function to compute memory footprint for a loop nest
- getMemRefRegion readability update - NFC

PiperOrigin-RevId: 230541857